A digital communication system typically transmits information or data using a continuous frequency carrier with modulation techniques that vary its amplitude, frequency or phase. After modulation, the signal is transmitted over a communication medium. The communication medium may be guided or unguided, comprising copper, optical fiber or air and is commonly referred to as the physical communication channel.
The information to be transmitted is input in the form of a bitstream which is mapped onto a predetermined constellation of symbols that defines the modulation scheme. The mapping of each bit as symbols is referred to as modulation.
A prior art base station is typically required to utilize multiple carriers converging continuous frequency spectrum. A block diagram of prior art superheterodyne receiver 11 which may be implemented in the base station is shown in FIG. 1. An operator is typically assigned two (2) or more channels Ch1–Ch4 (carriers), and desires to use them in each cell (frequency reuse=1). If this is not possible due to certain constraints which result in a frequency re-use factor that is lower, the operator has a finite number of channels, and will partition them in contiguous sections of spectrum so that a number of adjacent channels are used in each cell. In this case, the receiver 11 is required to process all channels (carriers) simultaneously. This minimizes hardware cost, size, and power consumption.
In the past, the high demanding requirements of base station receivers could only be met with a superhetrodyne architecture. The direct conversion architecture has many inherent problems that result from downconverting the RF signal directly to baseband. These problems include self-mixing which creates DC offsets in the baseband signal; even-order distortion which converts strong interfering signals to baseband; 1/f noise which is inherent in all semiconductor devices and which is inversely proportioned to the frequency (f) and which masks the baseband signal; and spurious emissions of the LO signal which interferes with other users. Direct conversion receivers also stress the state-of-the-art capabilities of the analog baseband processing components because gain control and filtering must all be done at baseband. This requires expensive amplifiers that possess high dynamic range and a wide bandwidth.
Conventional multi-carrier radios are based on a superheterodyne radio architecture that utilizes an intermediate frequency (IF) and direct digital sampling to block convert multiple carriers to and from baseband, as shown in FIG. 1 for the receiver. Because the IF is typically located above 50 MHz, direct digital sampling requires expensive high-speed or sub-sampling data converters, such as analog-to-digital converters (ADC) and digital-to-analog converters (DACs) capable of sampling rates greater than 100 MHz and requiring very low clock jitter.
Another disadvantage to direct digital sampling is the IF Surface Acoustic Wave (SAW) filters needed to reject interference in adjacent channels. The maximum number of carriers supported by the radio determines the bandwidth of the SAW filter. Support for a different number of carriers requires additional SAW filters. As an alternative, one IF filter can be used that covers the entire band of interest, but then additional dynamic range is needed in the ADC to handle the additional interference.
This can be understood from the dynamic range of the received signal. When the uplink channels are all under the control of the same base station, the radio frequency (RF) carriers will be received at similar power levels, requiring relatively less dynamic range in the ADC. However, if the IF filter bandwidth covers the entire band, uplink channels belonging to other base stations will be present at the input to the ADC. These channels can be at a very high level, thus requiring more dynamic range in the ADC.
Referring back to FIG. 1, the receiver 11 is used for digital multi-carrier wireless communication, for example a Code Division Multiple Access (CDMA) communication. As a signal is received at the antenna 15, it passes a first bandpass filter 16 and a linear amplifier 17. A second bandpass filter 18 receives the signal from the amplifier 17 and provides the signal to a mixer 19. A local oscillator 20 is connected to the mixer 19 and the mixer 19 translates the signal from RF to IF and is then filtered by a bandpass filter 21.
The bandpass filter 21 is connected to an ADC 22 which provides its digitized output to a digital downconverter 23. A complex numerically-controlled oscillator 24 is used to control the digital downconverter 23 to translate each channel at IF to baseband. The digital downconverter 23 provides quadrature baseband signals to a bank of finite impulse response (FIR) filters 25, which perform pulse shaping and interference rejection. The outputs from the FIR filters 25 are provided to respective digital automatic gain control circuits (DAGCs) 35 which provide outputs in four (4) respective channels 45. The digital data from each channel is sent to a digital processor (not shown) for further processing, such as data demodulation and decoding. Although four (4) channels are shown as an example, those of skill in the art would realize that there could be any number of channels.
A similar process is used on the transmission side, as shown in FIG. 2, which is a block diagram showing prior art transmitter 51 using four (4) input channels Ch1–Ch4 65. The four (4) input channels 65 are provided to respective power control circuits 75 which, in turn, provide their outputs to respective FIR filters 85. The FIR filters 85 are typically used for pulse shaping purposes. The outputs from the FIR filters 85 are provided in quadrature to a digital up converter 95, which is connected to a complex numerically-controlled oscillator 96. The output of the digital up converter 95 is provided to a digital-to-analog (DAC) circuit 97, which supplies its analog output to a first bandpass filter 98, which in turn is provided to an IF mixer 99. The IF mixer 99 receives its local oscillator signal from an oscillator 100 and provides an output to a second bandpass filter 102. The output bandpass filter is amplified at an amplifier 103, filtered at an output bandpass filter 104 and provided for transmission via antenna 105.
In these configurations (FIGS. 1 and 2), various conversions are performed with RF components. The manufacturing costs of these RF components is significant. Therefore, it would be advantageous to provide a circuit which avoids multiple RF conversions to the maximum extent practical. Additionally, a direct conversion design for a receiver and transmitter are desired.
The major problem with prior art direct conversion receivers is the generation of DC offsets at the output of the receiver. The major sources of DC offset are local oscillator self-mixing and second order intermodulation (IP2) of the mixer. DC offsets may be quite large, leading to saturation in the ADC and other performance problems in the receiver.
Solutions to the direct conversion problems have been understood for some time, but they were not practical or cost effective until recent technology developments made possible integrated solutions on monolithic RF integrated circuits (RFICs). These solutions to the problems include balanced (differential) structures that eliminate even-order distortion, SiGe semiconductor technology which exhibits low 1/f noise and excellent linearity, and harmonic mixing that eliminates self-mixing and LO spurious emissions. The move to wideband wireless technologies has also reduced the contribution of the 1/f noise to the overall noise floor of the direct conversion receiver. In addition, high-speed, high linearity amplifiers are now available to meet the analog baseband processing requirements.
However, there are still major problems with direct conversion receivers in the generation of DC offsets at the output of the receiver. The major sources of DC offset are LO self-mixing and second order intermodulation of the mixer. DC offsets may be quite large leading to saturation of the ADC and other performance problems in the receiver. Accordingly, although there have been advances with the prior art, these prior art techniques these still fall far short of the optimum performance.